1. Field of the Invention
The present invention relates to a method for forming a thin semiconductor film to be used for electronic or optical elements and more particularly a method for forming a thin semiconductor film with a controlled optical gap.
2. Description of the Prior Art
The thin films for electronic elements which have been formed by the thin film growing techniques such as the plasma chemical vapor deposition (plasma CVD) which mainly utilize energies except thermal energy has the problem that when the formed thin films are used in, for instance, optoelectronic transducers, the remarkable photo-induced degradation of their optoelectronic characteristics. On the other hand, in the case of thin films formed by thin film fabrication techniques which mainly utilize the thermal energy, such as the thermal CVD, their photo-induced degradation can be reduced to a minimum and the film can be grown in relatively uniform thickness over the irregularly shaped surface of a substrate.
Meanwhile it is well known to those skilled in the art that when the thin films for optoelectronic transducers such as amorphous silicon solar cells are fabricated by the thermal decomposition of silane compounds such as monosilane, disilane and so on, the thin films and elements with less optical degradation can be obtained. However, during the growth of a thin film the temperature of a substrate must be maintained at the order of about 500.degree. C. in practice. As a result, kinds of substrates used in thermal CVD are limited. Furthermore, the thin film quality is not satisfactory for the fabrication of the optoelectronic transducers so that the demand for improving the thin film quality is increased. So far it has been difficult to adjust an optical gap of thin films less than 1.6 ev.